Microstructured hollow core optical fiber using low chlorine concentration

ABSTRACT

The invention relates to an optical fiber having an axial direction and a cross section perpendicular to said axial direction, and a method and preform for producing such an optical fiber. The optical fiber is adapted to guide light at a wavelength λ, and comprises a core region, an inner cladding region surrounding said core region, and at least one of a first type of feature comprising a void and a surrounding first silica material. The core, the inner cladding region and the first type of feature extends along said axial direction over at least a part of the length of the optical fiber. The first silica material has a first chlorine concentration of about 300 ppm or less.

The invention relates to an optical fiber structure, a method of itsproduction, a preform for producing it, and use of the optical fiber.The optical fiber may be a Hollow Core (HC) Photonic Crystal Fiber (PCF)comprising a microstructured cladding arranged to provide a PhotonicBandgap (PBG). The optical fiber may be a solid core (SC) PhotonicCrystal Fiber (PCF) comprising a microstructured cladding arranged toprovide an effective index of the cladding below that of the core.Photonic Crystal Fibers are also referred to as Microstructured OpticalFibers or Holey Fibers. The PBG may be realized by arranging themicrostructured cladding in a manner, whereby it comprises a periodicmodulation of the refractive index. In a HC-PCF comprising a PBG, lightmay be guided in a core having an effective refractive index below theeffective refractive index of the surrounding material. The PBG guidingmechanism is fundamentally different from the total internal reflectionguiding mechanism, which is exploited in for example Large Mode Area(LMA) PCFs. In a HC-PCF guiding light by the PBG mechanism, a largefraction of the optical power may be present in the hollow core and inthe holes of the cladding, which result in a low power-in-glass fraction(<5%), which in turn may give rise to a very low non-linear coefficientand low loss due to intrinsic material absorption. Hollow and solid corePCFs may be fabricated by the stack-and-draw technique, where alarge-scale preform is prepared by stacking the elements of the preform,and subsequently drawing the preform to a fiber.

General methods of producing photonic crystal fibers are known from e.g.U.S. Pat. Nos. 7,321,712, and 6,985,661.

Typically, PCFs are produced using low loss silica material. Reductionof OH is one parameter for reducing losses of optical fiber forwavelengths of around 800 nm to beyond 2500 nm. Typically, low OHglasses have relatively high Chlorine content, as Chlorine is used toreduce OH. Examples of low loss silica materials include F300 and F500capillary tubes that are commercially available from Hereaus. F300 andF500 capillary tubes are for example used for optical fibers intelecommunications, amplifier and laser applications. Typical values ofOH contamination in F300 materials are 0.2 ppm.

The inventors of the present invention have realized that end facetdegradation in an optical fiber having voids, such as a HC-PCF or aSC-PCF, can be significant reduced if the diffusion of Chlorine fromsilica glass forming at least a part of the optical fiber is reduced.Unless this degradation is kept at a minimum, the end facet iscontaminated over time regardless of careful attempts to keep the endfacet free of e.g. dust particles. The inventors assign thecontamination to the formation of Chlorine containing compounds at theend facet when Chlorine from the silica glass surrounding the voids ofthe optical fiber diffuses into the voids and through these to the endfacet where Chlorine compounds are formed though e.g. reaction withwater in the ambient air. The inventors have realized that the Chlorinediffusion may be reduced by forming at least parts of the optical fiberfrom silica glass with a Chlorine concentration below 300 ppm or byarranging silica glass with a Chlorine concentration below 300 ppm at ornear the void surface of at least a part of the precursor elements ofthe preform, from which the optical fiber is produced.

One possible path for the formation of Chlorine compounds at the fiberend face includes the diffusion of Perchlorate ions from the silicamaterial through the voids to the fiber end facet, where they reactswith water in the ambient air forming crystalline hydrates. Analternative path may involve the formation of Ammonium Chloride salt atthe end facet. A microscope image of the end-facet of a HC fiber showingend facet degradation, which is believed to be related to Chlorinecontaining compound is shown in FIG. 1, where significant end facetdegradation is seen. The image is taken 25 hours after the fiber wascleaved and at it is evident, that the end facet is largelycontaminated.

In the context of the present invention, the phrase “Chlorineconcentration” refers to the atomic concentration regardless of whetherthe Chlorine is present as atomic Chlorine, Chlorine ions, molecularChlorine, or as part of a molecule with other elements, such asHydrochloric acid (HCl) or residual molecules originating from theformation of the silica glass of the optical fiber. In some cases, theChlorine concentration of a given silica material may be non-uniformover the cross section of the fiber preform or optical fiber. This mayfor instance occur when there is a descending Chlorine concentrationtowards the void of a first precursor element or a first type offeature. In such cases, the phrase Chlorine concentration refers to themaximum local concentration of Chlorine in said material as seen overthe cross section of the fiber preform or optical fiber. The phrasesconcentration and content are used interchangeably in this text.

The present inventors have realized that improved Photonic crystalfibers, including HC and SC fibers, can be realized using silicamaterials with low chlorine content, such as Chlorine content of lessthan 300 ppm.

In particular, the present inventors have realized that for wavelengthsin the range from 800 nm to at least 2500 nm, HC fibers made from glasswith low Chlorine content (and high OH content) has lower losses than HCfibers made from glass with high Chlorine content (and lower OHcontent)—despite the glass with high Chlorine having lower bulk glasslosses.

The present inventors have realized that a low Chlorine content is oneparameter for improving a HC fiber. The improvements relate to lowlosses (low attenuation) and low contamination in the fibers and/or atend-facets (end facet degradation) of the fibers.

The present inventors have further realized methods of producingimproved microstructured fibers, including HC PCFs and SC PCFs.

The improvements include lower fiber attenuation, reduced contaminationat fiber end-facets, reduced contamination at silica-air interfacesinside the fiber, reduction of amount of point scatters. Alternativeimprovements are also possible using the disclosed techniques andembodiments.

One object of the present invention is to provide an optical fiberadapted to guide light at a wavelength λ, said optical fiber having anaxial direction and a cross section perpendicular to said axialdirection. The optical fiber comprises a core region, an inner claddingregion surrounding said core region, and at least one of a first type offeature comprising a void and a surrounding first silica material. Thefirst silica material has a first attenuation coefficient, α₁, at λ anda first chlorine concentration, c₁, of about 300 ppm or less. The firsttype of feature extends along at least a part of the axial direction ofthe optical fiber.

One object of the invention is to provide a method for forming anoptical fiber adapted to guide light at a wavelength λ, said methodcomprising providing a fiber preform and drawing said fiber preform toform said optical fiber. The preform comprises a core part and an innercladding part arranged to provide a core region in the formed opticalfiber and a surrounding inner cladding region, respectively, at leastone of said parts comprising one or more precursor elements. The one ormore precursor elements comprises at least one of a first type ofprecursor element comprising a void and a surrounding first silicamaterial, said first silica material having a first attenuationcoefficient, α₁, at λ and a first chlorine concentration, c₁, of about300 ppm or less. The formed optical fiber having an axial direction anda cross section perpendicular to said axial direction.

The first silica material may be arranged to reduce the content ofChlorine or Chlorine compounds at or near the surfaces of the fiber endfacet and at the surfaces of the voids in said first type of feature orin the first type of precursor element.

One object of the present invention is to provide a fiber preform for aPhotonic Crystal Fiber, said fiber preform comprising a plurality oftubes arranged in a stack, wherein said tubes comprise a silica materialwith Chlorine content below 300 ppm. The plurality of tubes comprising aplurality of first type of features.

When the fiber preform is drawn to form an optical fiber, one first typeof precursor element may define one first type of feature in the formedoptical fiber, and the void in said first type of precursor element mayresult in the void in said first type of feature.

The fiber preform is drawn to an optical fiber using methods known theskilled person. The voids of the precursor elements may for example bepressurized during the drawing to control the void size of the firstand/or second type of features in the formed optical fiber.

One object of the present invention is to provide a Photonic CrystalFiber comprising a silica material with a plurality of elongated voidsextending along the longitudinal direction of the fiber, said silicamaterial having Chlorine content below 300 ppm. The Photonic CrystalFiber may have a hollow or a solid core, and said fiber being drawn froma preform according to the present invention.

One object of the present invention is to provide a material forproducing a Photonic Crystal Fiber, said material comprising a Silicaglass with Chlorine content of less than 300 ppm. The chlorine contentmay be about 250 ppm or less, such as about 200 ppm or less, such asabout 150 ppm or less, such as about 100 ppm or less, such as about 50ppm or less, such as about 25 ppm or less, such as about 10 ppm or less,such as about 5 ppm or less, such as about 1 ppm or less, such as about0.5 ppm or less, such as about 0.2 ppm or less, such as about 0.1 ppm orless, such as about 0.01 ppm or less, such as substantially free ofChlorine. The material may be produced in a method not including silicumtretra chloride. The PCF may be produced substantially from thismaterial or the material may be one of several used to produce the PCF.

In an optical fiber adapted to guide light at a wavelength λ, saidoptical fiber comprising a plurality of voids extending in thelongitudinal direction of the fiber, the optical fiber is improved byhaving at least a part of said voids surrounded by a first silicamaterial having a first chlorine concentration, c₁, of about 300 ppm orless.

In a fiber preform for forming an optical fiber adapted to guide lightat a wavelength λ, said fiber preform comprising a plurality of voidsextending in the longitudinal direction of the fiber, the fiber preformis improved by having at least a part of said voids surrounded by afirst silica material having a first chlorine concentration, c₁, ofabout 300 ppm or less.

The first chlorine concentration may be about 250 ppm or less, such asabout 200 ppm or less, such as about 150 ppm or less, such as about 100ppm or less, such as about 50 ppm or less, such as about 25 ppm or less,such as about 10 ppm or less, such as about 5 ppm or less, such as about1 ppm or less, such as about 0.5 ppm or less, such as about 0.2 ppm orless, such as about 0.1 ppm or less, such as about 0.01 ppm or less,such as substantially free of Chlorine.

In one embodiment, the core region of the optical fiber comprises atleast one of said first type of feature. The first type of feature maybe arranged to substantially form the core region and the optical fibermay be a hollow core fiber. In a hollow-core PCT, the core region may besurrounded by an inner cladding region comprising a plurality of thefirst type of feature or another type of feature. One way of realizing ahollow-core PBG fiber would be to have a first type of feature with alarger cross sectional diameter (the core region) being surrounded by aplurality of the first type of feature with a different cross sectionaldiameter, such as a smaller cross sectional diameter (the inner claddingregion), where the inner cladding provided a Photonic Bandgap in atleast one wavelength range. The hollow core may be surrounded by a solidinner cladding region, such as in a solid cladding Bragg fiber, wherealternating layers of material with different refractive indicesprovides the confinement of light to the hollow core and hence theguiding mechanism of the optical fiber.

In one embodiment, inner cladding region of the optical fiber comprisesa plurality of said first type of feature. This plurality of first typeof feature may be arranged in a periodic pattern such as a closed-packedhexagonal pattern resulting from arranging a plurality of first type ofprecursor elements in a closed-packed hexagonal arrangement in the fiberpreform as illustrated in FIG. 2. This Figure shows a schematicillustration of a section of a fiber preform 10 where a plurality offirst type of precursor elements 12 with voids 13 surrounds a core 11part defined by omitting 7 neighboring features. A closed packedhexagonal arrangement of prior art precursor elements is described forinstance in U.S. Pat. No. 6,985,661. The first type of features may alsobe arranged in a chain surrounding the core region as described in forinstance U.S. Pat. No. 5,907,652, wherein a high numerical aperture coreis surrounded by a ring of closely spaced voids separated by thin silicabridges. These bridges may be formed in said first silica material. Inprinciple may the plurality of first type of feature and any other typeof feature be arranged in any periodic or non-periodic pattern dependingon fabrication method and desired optical property of the optical fiber.

In one embodiment, the core region comprises a solid core. This may bethe case in a step index fiber, where the core may be surrounded by aninner cladding comprising a plurality of the first type of feature.

In one embodiment, the first type of feature further comprises a secondsilica material with a second attenuation coefficient, α₂, at λ andsecond chlorine concentration, c₂. The second attenuation coefficientmay be smaller than said first attenuation coefficient. The secondchlorine concentration may larger than said first chlorineconcentration.

The first silica material may be arranged to provide a diffusion barrierfor Chlorine situated in said second silica material, thereby mitigatingthe diffusion of said Chlorine into said voids of said first type ofprecursor element or said at least a first type of feature.

For at least a part of said plurality of the first type of precursorelement or feature, the first silica material may be arranged betweenthe void and the second silica material, i.e. the second silica materialsurrounds the first silica material and the void. Thereby the firstsilica material may form a Chlorine diffusion barrier, which may reducethe diffusion of Chlorine from the second silica material into the voidof the first type of precursor element or feature. A diffusion barrierin the first type of precursor element reduces the diffusion of Chlorineinto the void during the drawing of the optical fiber where the fiberpreform is maintained for some time at an elevated temperature. If thediffusion of Chlorine into the voids of the first type of precursorelement and accordingly into the void of a first type of feature, thereis less Chlorine which may form compounds at the surface of the voids orto diffuse though the voids to form Chlorine compounds there.Accordingly Chlorine induced end facet degradation may be mitigated.

In one embodiment, the first silica material is arranged to surroundsaid second silica material. In some fiber designs there is asignificant power-in-glass fraction in some parts of the optical fiber,such as in the core region and in the inner cladding region close to thecore region. The silica material, wherein the power-in-glass fraction issignificant is often in direct contact with the void, and it may bepreferred that silica material with a low attenuation at the wavelengthλ is arranged in direct contact with the void. Some low Chlorine contentsilica materials have a high attenuation due to the content of OH and itmay be preferable to arrange a layer of the second silica material(having a lower attenuation coefficient) in direct contact with thevoid, and accordingly accept a higher Chlorine content in the thisregion. This fiber design may still provide an advantage over prior artfiber designs in that the layer of second material may be relativelythin and the out diffusion of Chlorine from this region may besignificantly less than what would have been the case if all the silicamaterial have a high Chlorine concentration. The silica material (with ahigh Chlorine concentration) of prior art fibers may act as a Chlorinereservoir maintaining a high level of Chlorine out-diffusion for anextended period of time.

In one embodiment, the first type of feature may further comprise athird silica material arranged to surround said first and second silicamaterials, said third silica material having a third attenuationcoefficient, α₃, at λ and a third chlorine concentration, c₃. The thirdchlorine concentration may be smaller than said second chlorineconcentration.

The third chlorine concentration may be about 250 ppm or less, such asabout 200 ppm or less, such as about 150 ppm or less, such as about 100ppm or less, such as about 50 ppm or less, such as about 25 ppm or less,such as about 10 ppm or less, such as about 5 ppm or less, such as about1 ppm or less, such as about 0.5 ppm or less, such as about 0.2 ppm orless, such as about 0.1 ppm or less, such as about 0.01 ppm or less,such as substantially free of Chlorine

In one embodiment, the first silica material of the first type offeature is arranged in a substantially annular region with a firstthickness in the range of about 10 nm to about 5000 nm, such in therange of about 50 nm to about 4000 nm, such in the range of about 100 nmto about 3000 nm such in the range of about 200 nm to about 2000 nm,such in the range of about 300 nm to about 1000 nm, such in the range ofabout 400 nm to about 800 nm, such in the range of about 400 nm to about600 nm. In the context of the present application, the first thicknessis defined as the cross sectional dimension of the first silica materialalong a radial direction from the center of the void. When the crosssectional dimension of the layer of the first silica material is notuniform around the void, the thickness is defined as the average valueof the first silica material. Equivalent definitions may be defined forfurther silica materials such as the second and third silica materials.In one embodiment, the third silica material is arranged in asubstantially annular region with the third thickness being in the rangeof about 10 nm to about 5000 nm, such in the range such in the range ofabout 50 nm to about 4000 nm, such in the range of about 100 nm to about3000 nm such in the range of about 200 nm to about 2000 nm, such in therange of about 300 nm to about 1000 nm, such in the range of about 400nm to about 800 nm, such in the range of about 400 nm to about 600 nm.The thickness of the first and third silica materials in the opticalfiber may be determined from the thickness of the corresponding layersin the precursor elements of the fiber preform from which the opticalfiber is formed.

In one embodiment, the first silica material of the first type ofprecursor element is arranged in a substantially annular region with athickness which after drawing of the optical fiber results in athickness in the range of about 10 nm to about 5000 nm, such in therange of about 50 nm to about 4000 nm, such in the range of about 100 nmto about 3000 nm such in the range of about 200 nm to about 2000 nm,such in the range of about 300 nm to about 1000 nm, such in the range ofabout 400 nm to about 800 nm, such in the range of about 400 nm to about600 nm.

In one embodiment, the first type of precursor element comprises a thirdsilica material which may be arranged in a substantially annular regionwith a thickness which after drawing of the optical fiber results in athickness in the range of about 10 nm to about 5000 nm, such in therange of about 50 nm to about 4000 nm, such in the range of about 100 nmto about 3000 nm such in the range of about 200 nm to about 2000 nm,such in the range of about 300 nm to about 1000 nm, such in the range ofabout 400 nm to about 800 nm, such in the range of about 400 nm to about600 nm.

FIG. 9 shows a schematic illustration of a section of a fiber preform 90where a plurality of first type of precursor elements 92 with voids 93surrounds a core 91 part defined by omitting 7 neighboring features. Thefirst type of precursor elements may comprise first silica material 94surrounded by second silica material 95, which may be surrounded bythird silica material 96. In this embodiment, the first and the thirdsilica materials may define a diffusion barrier for chlorine in saidsecond silica material.

The thicknesses in the precursor elements may preferably be such thatthe diffusion of Chlorine and Chlorine containing compounds to thesurface of the voids of the precursor elements during the drawing of theoptical fiber is reduced significantly. The use of silica material witha Chlorine concentration of about 300 ppm or less in the fiber preformand accordingly in the optical fiber may result in a reduction in thecontent of Chlorine and Chlorine containing compounds in the voids andat their surfaces corresponding to about 50% or more, such as about 60%or more, such as about 70% or more, such as about 80% or more, such asabout 90% or more, such as about 95% or more, such as about 98% or more,such as about 99% or more, such as about 99.9% or more compared to casewhen silica material with a Chlorine concentration of about 300 ppm ormore is used.

In one embodiment, the first and the third silica materials aresubstantially identical.

In one embodiment, the optical fiber further comprises a second type offeature comprising a void and a silica material surrounding this void,said first and second type of feature being different in at least thesilica material surrounding their respective voids. The difference inthe silica material may relate to the composition of the silica materialthat is in direct contact with the voids of the first and second type offeatures. The attenuation coefficient at λ of the material in directcontact with the void of said second type of feature may be smaller thanthe first attenuation coefficient. The silica material of said secondtype of feature may be comprised substantially of a silica material witha Chlorine content of about 300 ppm or more. Such Chlorine content isoften the result of the cleaning process aiming to reduce the OH contentin the silica glass.

In one embodiment, the silica material surrounding the void in saidsecond type of feature is substantially identical to said second silicamaterial.

In one embodiment, said core region comprises at least one of saidsecond type of feature. The optical fiber may be a hollow core fiber,where the void of the second type of feature forms the hollow core. Thisdesign may be used in configurations, where the guiding properties ofthe hollow core requires or benefits from having the feature definingthe core region formed substantially in silica material with anattenuation coefficient or a OH content being different from that of thefirst silica material. In such an embodiment, the optical fiber stillpresents an improvement over prior art optical fibers if the innercladding comprises a plurality of said first type of features.

The inner cladding region may comprise a plurality of said first type offeature and a plurality of said second type of feature. In oneembodiment, the first and second type of features are arranged so thatthe part of the inner cladding region closest to the core regioncomprises a majority of said first type of feature. This arrangement mayfor instance be preferred when the Chlorine induced end facetdegradation is to be kept at a minimum close to the core, while it isless important to avoid the formation of Chlorine containing compound atthe end face parts not in the immediate vicinity of the core.

In one embodiment, the first and second type of features are arranged sothat the part of the inner cladding region closest to the core regioncomprises a majority of said second type of feature. This arrangementmay for instance be preferred when the often higher attenuation in thefirst silica material is considered to be a potential problem for theguiding properties of the optical fiber, and the presence of the firsttype of feature close to the core where the power in glass fraction maybe high should be avoided or kept at a minimum. In one embodiment, thecore may be surrounded by e.g. one or two rings comprising a majority ofsecond type of feature, and further surrounding these rings are thenplaced one or more rings comprising a majority of first type of feature.Thereby the presence of first silica material in parts of the innercladding regions with a high power in glass fraction is kept at aminimum while end facet degradation still is mitigated by having a partof the inner cladding region being comprised of a plurality of firsttype of feature.

In one embodiment, the optical fiber further comprises an outer claddingregion surrounding said inner cladding region, said outer claddingregion comprising an outer cladding silica material. The outer claddingsilica material may be substantially identical to said first silicamaterial. The outer cladding silica material may be substantiallyidentical to said second silica material or in principle be any silicamaterial suited for the production of silica based optical fibers.

In one embodiment, the inner cladding part of the fiber preformcomprises a plurality of precursor elements arranged in a substantiallyperiodic arrangement in said cross section. The periodic arrangement maycomprise a closed packed hexagonal structure.

In one embodiment, the method is adapted to form a hollow core opticalfiber. A hollow core optical fiber may be formed from a preformcomprising a substantially periodic arrangement of precursor elements inthe inner cladding part, and the core part may be defined by removing atleast one precursor element from this periodic arrangement of precursorelements. In one embodiment, 7 or 19 precursor elements are removed fromthis periodic arrangement of precursor elements to define the core part.In principle may any number of precursor elements be removed from theperiodic arrangement to e.g. provide a hollow core which is elongated inthe fiber cross section.

In one embodiment, the inner cladding part comprises a plurality offirst type of precursor element. If the first type of precursor elementsare arranged in a closed packed hexagonal structure, the correspondingplurality of first type of feature in the inner cladding region willdescribed a triangular pattern as seen in Large Mode Area PhotonicCrystal Fibers and in some hollow core Photonic Bandgap Fibers. In oneembodiment, the prefrom according to the present invention comprisestubes arranged in a manner whereby a Hollow-core Photonic Crystal Fiberor a Solid-Core PCT can be drawn from the preform. The core part mayalso be formed by a first type of precursor element with a crosssectional dimension which is larger than the cross section dimension ofthe plurality of first type of precursor element arranged in the innercladding part. The core part may also be formed by a first type ofprecursor element which is pressurized to a higher level than theplurality of first type of precursor element arranged in the innercladding part, whereby cross sectional dimension of the first type offeature in the core region is larger than the cross section dimension ofthe first type of feature in the inner cladding region.

In one embodiment, first type of precursor element further comprises asecond silica material having a second attenuation coefficient, α₂, at λand a second chlorine concentration, c₂. The second silica material maybe arranged to surround said first silica material or the first silicamaterial may be arranged to surround said second silica material.

In one embodiment, the first type of precursor element comprises a thirdsilica material having a third attenuation coefficient, α₃, at λ and athird chlorine concentration, c₃, of about 300 ppm or less, wherein saidthird silica material is arranged to surround said first and secondsilica materials.

One object of the present invention is to provide a method of reducingammonium chloride contamination in Photonic Crystal Fiber, said methodcomprising providing a stack of first type of precursor elements to forma Photonic Crystal Fiber preform, wherein said first type of precursorelements are made from silica glasses with a Chlorine content of lessthan 300 ppm. The Chlorine content in the silica may be about 250 ppm orless, such as about 200 ppm or less, such as about 150 ppm or less, suchas about 100 ppm or less, such as about 50 ppm or less, such as about 25ppm or less, such as about 10 ppm or less, such as about 5 ppm or less,such as about 1 ppm or less, such as about 0.5 ppm or less, such asabout 0.2 ppm or less, such as about 0.1 ppm or less, such as about 0.01ppm or less, such as substantially free of Chlorine. The optical fiberformed using this method may be a Hollow-Core Photonic Crystal Fiber ora Solid-Core Photonic Crystal Fiber. The precursor elements may also bereferred to as capillary tubes.

A method of reducing ammonium chloride contamination in silica basedPhotonic Crystal Fiber may comprise providing Hydrogen ions to thesilica material to form Hydrogen Chloride molecules with the Chlorinepresent in said silica material or the Chlorine forming compounds at thesurface of the voids. The hydrogen may be provided to the silicamaterial by high pressure Hydrogen loading, which may be performed at ahydrogen pressure of about 1 to about 2000 bars, such as of about 10 toabout 1500 bars, such as of about 25 to about 1000 bars, such as ofabout 50 to about 500 bars, such as of about 100 to about 300 bars.Out-diffusion of said Hydrogen Chloride following its formation isaccelerated by heating the silica material in e.g. a bake. The hydrogenloading may be performed to the preform prior to the drawing of theoptical fiber or to the produced optical fiber. Deuterium may be usedinstead of hydrogen.

The capillary tubes are sealed using a flame from for examplehigh-purity hydrogen and oxygen. In one embodiment, said sealed ends arecleaned by an etching process, such as etching by using a HydrogenFluoride etch.

The silica material may be selected from the group of natural occurringquartz and thermal oxide glass.

One object of the present invention is to provide a method of producinga preform for a Photonic Crystal Fiber comprising:

-   -   a. providing a number of capillary tubes made from a silica        material;    -   b. optionally seal at least a part of said capillary tubes in        one or both ends;    -   c. HF-wash at least a part of said capillary tubes;    -   d. rinse and dry at least a part of said capillary tubes;    -   e. assemble a stack from said capillary tubes to form a HC fiber        perform;        wherein the Chlorine content in the silica material of said        capillary tubes is reduced to below 300 ppm during steps b to d.

In the following, the design and application of an optical fiberaccording to the invention will be discussed in connection withexamples.

BRIEF DESCRIPTION OF DRAWINGS

The invention will be explained more fully below in connection with apreferred embodiment and with reference to the drawings in which:

FIG. 1 shows end facet degradation in a Hollow-Core PCF (HC-PCF),

FIG. 2 shows a schematic illustration of a section of a fiber preformwith a plurality of first type of precursor elements in a closed-packedhexagonal arrangement,

FIG. 3 shows a schematic illustration of a preform for the production ofa HC-PCF comprising a stack of capillary tubes and an overcladding tube,

FIG. 4 shows the end facet of an optical fiber according to the presentinvention,

FIG. 5 shows the end facet of an optical fiber according to the presentinvention

FIG. 6 shows the end facet of an optical fiber according to the presentinvention imaged over several hours,

FIG. 7 shows the end facet of an optical fiber according to the presentinvention and the attenuation of a mode propagating in the core,

FIG. 8 shows the measurement of HCl in the hollow core of a prior artfiber, and

FIG. 9 shows a schematic illustration of a section of a fiber preformwith a plurality of first type of precursor elements in a closed-packedhexagonal arrangement, together with two different embodiments of thefirst type of precursor element.

The figures are schematic and may be simplified for clarity. Throughout,the same reference numerals are used for identical or correspondingparts.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

The invention is defined by the features of the independent claim(s).Preferred embodiments are defined in the dependent claims. Any referencenumerals in the claims are intended to be non-limiting for their scope.

Some preferred embodiments have been shown in the foregoing, but itshould be stressed that the invention is not limited to these, but maybe embodied in other ways within the subject-matter defined in thefollowing claims.

In the following examples to further illustrate preferred embodiments ofthe present invention(s) are described.

FIG. 3 shows a schematic illustration of a preform 20 for the productionof a HC-PCF comprising a stack of precursor elements 22 each comprisinga void 23, and an overcladding tube 25. The core part comprises aprecursor element 24 with a void 21. Both the precursor elements of thecore and the inner cladding region may be of the first type of precursorelement. Additional elements may be included in the stack, such as forexample solid rods at interstitial sites in the cladding, a hollow tube(in the center) and filling rods to for example assist mechanicallystability of the stacked perform. The figure is schematic and simplifiedfor clarity, and just shows details, which ease the understanding of theinvention, while other details are left out.

The use of silica material with a Chlorine content of about 300 ppm orless drastically reduces the amount of contamination as illustratedusing FIG. 4 and FIG. 5.

In FIG. 4 was taken 4 days after the cleaving of the fiber and onlysub-micron sized droplets are forming on the glass part of theend-facet. The end facet contamination is clearly mitigated when usingsilica materials with a very limited content of Chlorine (<300 p.p.m.)for the precursor elements. This could for example be F100/F110 glassfrom HERAEUS or crystalline SiO₂ from commercial vendors. On FIG. 5 isshown a microscope image of the end facet of a HC fibers build formcapillary tubes drawn from F110 glass taken 4 days after the cleaving ofthe fiber. The use of this material effectively cancels the building upof end facet contamination over time. As it was also concluded above, itis highly plausible the facet contamination is related to the presenceof Cl in the glass material constituting the fiber.

FIG. 6 shows measurement results for another optical fiber producedusing a preferred embodiment of the present invention, where images ofthe end facet are taken several hours after cleaving the optical fiber.The largest image showing the end facet 100 hours after cleaving

The inventors have shown that PCFs can be produced in silica materialwith a low Chlorine content. FIG. 7 shows the end facet of an opticalfiber according to the present invention and the attenuation of a modepropagating in the core. At 1550 nm the attenuation 14.0+/−0.7 dB/km,while the lowest attenuation of 13.2+/−0.4 dB/km is obtained at 1504 nm.The 10 dB Band gap Width is 173 nm (1654-1481) and the number of scatterpoints is 1 km⁻¹.

The present inventors have realized methods for monitoring orcharacterizing the amount of gasses in hollow core fibers. Preferably,the methods are used for sensor and/or measuring applications. In FIG. 8is shown a measurement of the HCl content in the hollow core clearlyindicating the this Chlorine compound is present in the core region ofthe fiber and diffusion of Chlorine to the fiber and may be the reasonfor end fact degradation in optical fibers with a Chlorine content ofmore than 300 ppm. The absorption peaks measured on the hollow corefibres most probably stems from the first overtone of the HCl rotationsand vibrations.

The present inventors have realized that further improvements to HCfibers may be obtained using for example bake out of preforms and/orimprovements to the fiber drawing process. These improvements includeflushing preform with gasses, such as Oxygen, Ar, or other types ofinert gasses. In particular, flushing preforms before, during or afterfibers drawing is advantageous. In particular, it is preferred thatflushing with gasses having low N₂ level in order to reduce or eliminateAmmoniumhydroxide at fiber end facets and/or inside fibers with holes.

The reaction to generate Ammoniumhydroxide may be written as:NH₃+H₂O→NH₄ ⁺OH⁻(water solution)NH₄ ⁺OH⁻+HCl→NH₄Cl+H₂O

Notice that Ammoniumhydroxide has a boiling point of 38-100° C.(reference: Sigma-Aldrich).

Typically, all substances are introduced or appearing during productionof in HC fibers.

In some embodiments, the Chlorine level in the silica glass is kept at alow level by avoiding the used of Chlorine cleaning and/or using silicathat has not undergone Chlorine cleaning steps at manufacture, Nitrogenduring pressure control is avoided (Helium may be used as flush gas onstack to avoid Nitrogen in the stack), Argon gas may be used forpressure control in process steps, such as fiber drawing process step,and the water content in the silica is kept at a low level bycontrolling gas composition during process steps (sealing and flushingprior to heating steps).

A gas flow, such as Argon flow, through the cane may be used.Optionally, a bake-out before drawing a fiber from the preform is made.

In further embodiments, cold traps are used. For example, peltierelements and/or dry ice is preferred to liquid Nitrogen to avoidpressure instabilities in pressure control using either Nitrogen orArgon.

The method according to the present invention may furthermore comprisethe steps of a Bake-out and/or a flushing with a gas selected from thegroup of Ar, O₂, He, Ne, Kr, or Xe.

The present invention is not limited to specific PCF designs, but may beutilized in general to produce any kind of optical fibers comprising oneor more voids. The various preferred embodiments and improvements may beused independently or in any combination.

The invention is applicable to transmission systems, gyroscopes, andsensors in general, gas lasers, and lasers and amplifiers in general,pulse compression, dispersion compensation, but it is not limited tosuch uses.

Some embodiments have been shown in the foregoing, but it should bestressed that the invention is not limited to these, but may be embodiedin other ways within the subject-matter defined in the following claims.Various modifications and applications may occur to those skilled in theart without departing from the true spirit and scope of the invention asdefined in the appended claims

The invention claimed is:
 1. A microstructured optical fiber adapted to guide light at a wavelength λ in the range from 800 nm to 2500 nm, said optical fiber having an axial direction and a cross section perpendicular to said axial direction, said optical fiber comprising: a hollow core region, an inner cladding region surrounding said core region, an end facet at an end of the optical fiber, and at least a portion of the optical fiber is comprised of a first type of feature comprising a void and a surrounding first silica material, said first type of feature extending along at least a part of said axial direction, said first silica material having a first attenuation coefficient, α₁, at λ and a first chlorine concentration, c₁, of about 300 ppm or less, providing that diffusion of chlorine to the end facet and resulting chlorine-induced degradation of the fiber end facet is mitigated.
 2. The optical fiber according to claim 1, wherein said core region is formed by the void of one of said first type of feature.
 3. The optical fiber according to claim 1, wherein said inner cladding region comprises a plurality of said first type of feature.
 4. The optical fiber according to claim 1, wherein said first type of feature further comprises a second silica material with a second attenuation coefficient, α₂, at λ and second chlorine concentration, c₂.
 5. The optical fiber according to claim 4, wherein said second attenuation coefficient is smaller than said first attenuation coefficient.
 6. The optical fiber according to claim 4, wherein said first silica material is arranged to surround said second silica material.
 7. The optical fiber according to claim 4, wherein said second silica material is arranged to surround said first silica material.
 8. The optical fiber according to claim 4, wherein said first type of feature further comprises a third silica material arranged to surround said first and second silica materials, said third silica material having a third attenuation coefficient, α₃, at λ and a third chlorine concentration, c₃.
 9. The optical fiber according to claim 8, wherein said first and third chlorine concentrations are smaller than said second chlorine concentration.
 10. The optical fiber according to claim 4, wherein said first silica material is arranged to provide a diffusion barrier for Chlorine situated in said second silica material, thereby mitigating the diffusion of said Chlorine into said voids of said first type of feature.
 11. The optical fiber according to claim 1, wherein said first silica material is arranged in a substantially annular region with a thickness in the range of about 10 nm to about 5000 nm.
 12. The optical fiber according to claim 1, further comprising a second type of feature comprising a void and a silica material surrounding this void, said first and second type of feature being different in at least the silica material surrounding their respective voids.
 13. The optical fiber according to claim 12, wherein said inner cladding region comprises a plurality of said first type of feature and a plurality of said second type of feature, wherein the first and second type of features are arranged so that the part of the inner cladding region closest to the hollow core region comprises a majority of said second type of feature.
 14. The optical fiber according to claim 12, wherein said difference in the silica material relates to the composition of the silica material that is in direct contact with the voids of the first and second type of features.
 15. The optical fiber according to claim 12, wherein said core region comprises at least one of said second type of feature.
 16. The optical fiber according to claim 12, wherein said inner cladding region comprises a plurality of said first type of feature and a plurality of said second type of feature, wherein the first and second type of features are arranged so that the part of the inner cladding region closest to the core region comprises a majority of said first type of feature.
 17. The optical fiber according to claim 1, wherein the voids in said first type of feature have a surface and said first silica material is arranged to reduce the content of Chlorine or Chlorine compounds at or near the surfaces of the voids in said first type of feature.
 18. The optical fiber according to claim 1, wherein said fiber is adapted to guide light at a wavelength λ in the range from 1481 nm to 1654 nm.
 19. The optical fiber according to claim 1, wherein the silica glass is substantially free of chlorine.
 20. The optical fiber according to claim 1, wherein said inner cladding comprises a plurality of the first type of feature, said first silica material of the plurality of first type of feature mitigates diffusion of chlorine into the voids of the plurality of the first type of feature, thus reducing migration of the chlorine to the end facet.
 21. A hollow core microstructured optical fiber adapted to guide light at a wavelength λ in the range from 800 nm to 2500 nm, said optical fiber comprising a plurality of voids extending in the longitudinal direction of the fiber, wherein said optical fiber is improved by having at least one of said voids surrounded by a first silica material having a first chlorine concentration, c₁, of about 300 ppm or less.
 22. The optical fiber according to claim 21, further comprising an end facet at an end of the optical fiber.
 23. The optical fiber according to claim 22, wherein for at least a part of said voids, said first silica material is arranged between the void and a second silica material having a second chlorine concentration, c₂, which is larger than said first chlorine concentration, so that said first silica material forms a Chlorine diffusion barrier arranged to reduce diffusion of Chlorine from said second silica material into the void, whereby diffusion of Chlorine to the fiber end facet and accordingly Chlorine induced end facet degradation is mitigated.
 24. The optical fiber according to claim 21, wherein said fiber is adapted to guide light at a wavelength λ in the range from 1481 nm to 1654 nm. 